Tidal deformability in neutron stars from a microscopic point of view

TL;DR

This work assesses tidal deformability in neutron stars using a microscopically grounded EoS based on chiral EFT, incorporating high-density continuations via speed-of-sound parametrizations and, alternatively, a conformal-limit approach. The authors compute the tidal deformability $\Lambda$, tidal Love number $k_2$, and the effective tidal deformability $\tilde{\Lambda}$ by solving the TOV equations with linear perturbations, obtaining $\Lambda_{1.4} = 355.25 \pm 85.64$ and $R_{1.4} = 11.99 \pm 0.51$ km for their representative case, with $\tilde{\Lambda}$ constrained to $(197,720)$ by GW170817. They show their predictions lie within multimessenger bounds, ruling out stiff EoS with radii larger than about 13.2 km, and find that the tidal observables are primarily sensitive to the mid-density regime, with the maximum-mass result constraining the high-density extension. The study emphasizes controlled EoS extensions to connect ab initio nuclear theory with gravitational-wave observations and highlights the ongoing value of EoS-independent measurements to refine neutron-star radii.

Abstract

We present results for the tidal deformability in neutron stars, the tidal Love number $k_2$, and the effective deformability of a binary system. The microscopic equation of state for cold $β$-stable neutron matter is based upon high-precision two-neutron forces and includes the chiral three-neutron forces required at the chosen order. We review and motivate our choices for the high-density continuation of the microscopic equation of state. We discuss our predictions and observe that they are well within multimessenger constraints. In contrast, stiff equations of state that yield radii larger than about 13 km are ruled out by GW170817 constraints.

Figures (7)

• Figure 1: Left: M(R) curves at fourth order (N$^3$LO, blue) and at third order (N$^2$LO, red) of ChPT. Dashed curves: the first extension is done using a polytrope with $\gamma$ = 3.3, followed by pressure values given by Eq. (\ref{['pi']}) together with Eq. (\ref{['vs']}); Solid curves: obtained with $\gamma$ = 3.8, a value beyond which the EoS begins to violate causality. Right: Dimensionless speed of sound squared corresponding to the curves on the left. Same color and pattern conventions. The figure is taken from Ref. S+A_2025.
• Figure 2: Left: the red and blue M(R) curves are as described in Fig. \ref{['7+vs']}. Green curves: M(R) relations at N$^3$LO (dashed) and at N$^2$LO (solid) where the polytropic part of the EoS has adiabatic index equal to 3.5, and the last continuation is based on the conformal speed of sound parametrization, Eq. (\ref{['skew']}). Right: Dimensionless speed of sound squared corresponding to each of the cases displayed on the left. See text for additional explanation.
• Figure 3: Left: The dimensionless tidal deformability as a function of the neutron star mass (left) and as a function of the compactness (right). The pink dots and their uncertainties are taken from the Bayesian analysis of Ref. LH19. See text for more details.
• Figure 4: Left: The tidal Love number as a function of the neutron star mass (left) and as a function of the compactness (right).
• Figure 5: Pressure profile in neutron stars with different masses as given in the inset.